In recent years the aviation industry has been involved in a seemingly never ending quest to enhance the performance of modern avionics. One particular area in which avionics engineers have expended much effort and achieved many successes is in the substitution of completely electro-optical systems for the conventional mechanical systems of the past. A prime example of these efforts is in the area of inertial measurement units (IMUs).
Gyroscopes have been utilized as IMUs to provide information concerning the orientation of the airplane with respect to a reference plane, typically the ground. State of the art avionics control systems utilize solid state fiber optic ring gyroscopes (a.k.a. FOG) to provide angular rate information regarding changes in the plane's orientation. Fiber optic ring laser gyros typically have as one of their components an optical readout device for analyzing the frequency of light signals that are split off from the ring. Often this is done with a fiber optic directional coupler which is connected to an optical detector by two optical fibers. For example, U.S. Pat. No. 4,913,548 describes such a solid state fiber optic gyro which has a reduced relative size and weight and which is capable of detecting both the magnitude and direction of the angular rotation of the fiber optic gyro by comparing signal phase information.
The global positioning system (GPS) satellite navigation systems provides a worldwide position, velocity and time reference available for utilization in civilian aviation electronic navigation systems. The civilian global positioning system service has allowed accurate positioning information processing apparatuses to be incorporated into low cost consumer electronics such as avionics navigation systems. Further, the global positioning system has been approved for sole means navigation making GPS an attractive alternative to traditional avionics systems.
In avionics navigation and control systems, it is highly desirable to incorporate redundancy in order to provide integrity and control of the airplane. IMU failure detection presently comprises physical redundancy by using multiple similar sensors. However, increasing system redundancy directly increases the cost of the navigation system. Redundancy necessitates adding at least one more sensor than is required for fault free operation. Typically, it would be desirable to provide an aircraft with two or three individual attitude and heading reference systems (AHRS) to provide angular movement information in three dimensions (3D). Small aircraft customers, however, cannot afford to carry spare equipment, but still want or need a redundant reference for gyroscope fault detection and isolation.
With three attitude and reference heading systems, a failed AHRS system can be detected and isolated. With two attitude and reference heading systems, a failed AHRS system can be detected but the failed unit cannot be isolated. It is therefore desirable to provide a less costly means for failure detection and isolation.